The objective of this study is to study the IgE-meidated signals which stimulate basophils and rat mast cells to undergo desensitization. A second objective is to study the biochemical mechanisms by which the desensitizations induced and maintained. subthreshold desensitization is the preferred model for these studies since minimal IgE cross-linking induces desensitization without histamine release. We will study the degree and specificity of desensitization of human basophils and rat mast cells by determining the effects of the concentration of desensitizing and rechallenge antigens, as well as the number and antigen-specificity of the IgE. Rat mast cells will be studied to determine biochemical correlates of desensitization including: calcium fluxes, changes in cyclic nucleotides and phospholipids.